Spinning has been defined as the transformation of a liquid material into a solid fibre. There are three main methods for spinning fibres: melt spinning, dry spinning and wet spinning. These methods can be combined depending on the final properties required of the material (such as a polymer) being spun.
Melt spinning is preferred if the polymer can be melted without degradation and is a common method for spinning thermoplastics such as polypropylene and nylon. The molten polymer is extruded through a spinneret into a gaseous medium such as air where the fibre cools producing solid, non-porous fibre. The filament is usually then drawn to orientate the polymer molecules which also improves the tensile properties of the fibre.
Dry spinning involves the extrusion of a polymer dope (polymer dissolved in an appropriate solvent) into a heated zone where the solvent evaporates. This is a slower process than the cooling of melt spun fibres and, as a result tends to produce fibres with non-uniform properties and a less circular cross section.
Wet spinning is identical to dry spinning except in the way the solvent is removed from the extruded filaments. Instead of evaporating the solvent, the fibre is spun into a liquid bath containing a solvent/non-solvent mixture called the coagulant. The solvent is nearly always the same as that used in the dope and the non-solvent is usually water.
Dry and wet spinning can be combined to form a process known as dry jet wet spinning. Polymer dissolved in a suitable solvent is extruded into a gap before entering a coagulation bath containing a coagulant that is miscible with the solvent but not with the polymer. A phase inversion process takes place producing a solid fibre. The bath can contain a mixture of solvent and non-solvent. This method helps prevent blockage of the spinneret and also allows some drawing of the fibre prior to coagulation, increasing orientation of the polymer molecules. The air gap has been shown to produce fibres that are stronger and more extensible than fibres produced from an immersed jet.
The fibre microstructure is established in the coagulation bath and requires optimisation of conditions. The critical process is the transition from a liquid to a solid phase within the fibrils and there are two possible such transitions. One is phase inversion--the precipitation of polymer to form a solid phase, the other is gelation. The former yields fibre of poor mechanical properties where as the latter produces an elastic gel giving rise to a fine microstructure once the solvent is removed. For membrane-type fibres phase inversion is preferable. For fibres with the appearance of a solid wall phase inversion should be slowed down so that gelation precedes phase inversion. Conditions in the coagulation bath have, therefore, to be optimised so that gelation precedes phase inversion. It has been shown that gelation occurs more rapidly at lower temperatures and at higher solid concentration in the dope.
The concentration of solvent in the coagulation bath can also be adjusted to obtain the desired microstructure. A low solvent concentration promotes rapid solvent extraction although this results in a thick skin on each filament which ultimately reduces the rate of solvent extraction and can lead to the formation of macrovoids. A high concentration of solvent in the coagulant gives a denser microstructure but solvent extraction is low. Temperature of the coagulation bath, jet stretch and immersion bath can similarly affect coagulation and microstructure. The fibre produced is essentially a swollen gel and is unoriented. The microstructure consists of a fibrilar network with the spaces in-between called macrovoids.
The invention is directed towards an improved spinning method of dry-jet wet spinning which enables the production of hollow polymeric fibres with the hole or lumen accurately centred and permits an enhanced degree of control over the wall properties. Consistent wall properties are likely to be of great significance in a range of applications: for example the best combination of tensile properties is achieved when the fibre has a homogeneous, dense gel structure with small fibrils and no macrovoids; for application as a membrane the wall ideally has a highly oriented inner and outer skin separating a porous body. The invention is also directed towards a suitable spinning apparatus; in particular one which is suitable for the production of polyacrylonitrile fibres suitable for subsequent processing to produce hollow carbon fibres.
According to an aspect of the invention a method of manufacture of hollow polymeric fibres comprises the steps of:
i) dissolving polymer in a suitable solvent to form a dope; PA1 ii) extruding the dope through an aperture in a spinneret to form a jet of liquid; PA1 iii) injecting a first coagulant into the centre of the dope jet as it leaves the spinneret; PA1 iv) directing the jet through an air gap into a coagulant bath containing a second coagulant such that a fibre is formed; PA1 v) directing the fibre through a drawing bath to reduce the diameter;
wherein each coagulant comprises a mixture of a coagulant liquid capable of causing gelation and eventual solidification of the dope jet and between 20% and 80% of the solvent liquid.
The invention produces hollow fibres whilst allowing a high degree of control over the spinning conditions and thus over the structure of the fibre wall. In particular for fibres with the appearance of a solid wall phase inversion should be slowed down so that gelation precedes phase inversion. The hollow fibres thereby produced offer comparable tensile properties at reduced weight in comparison to solid fibres produced by conventional wet spinning, offering advantages in a range of applications such as in the production of hollow fibres for textiles. It will be understood that the invention is not limited to production of single fibres but can produce multiple fibre arrays from multiple liquid jets either by providing a spinneret with multiple apertures or by providing an array of spinnerets.
Carbon fibres are manufactured by pyrolysing organic precursor fibres, predominantly polyacrylonitile (PAN) fibres produced by wet spinning. It may be noted here that the polyacrylonitrile fibre is used in this art to include co-polymers or ter-polymers of acrylonitrile with other monomers. For precursors of carbon fibre this is typically a copolymer with itaconic acid which controls the cylcisation reaction during pyrolysis. The requirement that gaseous products must be able to diffuse through the fibres from the surface to the centre, and vice-versa during the oxidation and carbonisation processes, imposes an upper diameter limit and the technique is limited to the production of carbon fibres for structural applications with diameters up to about 10 .mu.m.
In the last decade, the tensile strength of these fibres has been doubled, leading to large increases in all tensile-related composite properties. However, under compressive loading the failure process is micro-buckling. Compressive strength is therefore strongly influenced by the diameter limit set by the manufacturing process and has remained largely unchanged over this period. As a result this property is often the key design parameter in strength critical applications. Hollow carbon fibres offer a possible solution as they offer the potential for increased second moment of area and hence resistance to buckling without exceeding thickness limits. This would require production of hollow precursor fibres of an appropriate size, and with a dense walled structure without macrovoids.
The invention is thus particularly applicable to the production of acrylic fibres such as polyacrylonitrile to serve as hollow carbon fibre precursors. Polyacrylonitrile of molecular weight in the range 80,000 to 200,000, typically about 120,000 is preferred, and is dissolved in an appropriate aprotic solvent, of which dimethyl formamide (DMF) and sodium thiocyanate are non-limiting examples. The dope formed preferably contains between 15% and 30% by weight, and typically 25%, by weight of polyacrylonitrile in the appropriate solvent. A preferred coagulant is water. The polymer concentration in the dope solution is preferably in the range 15-25%. The solvent concentration in the coagulant solution is preferably in the range 30-60%.
There is also the potential to incorporate a third phase into the hollow fibre core after formation which could find application in the smart materials field. For example, uncured resin could provide in-situ repair capability after fibre fracture or suspensions of fine powders could act as radar absorbers for stealthy capability.
Hollow carbon fibres suitable for applications where conventional carbon fibres are used at present will have diameters in the preferred range 20-40 .mu.m, corresponding to polyacrylonitrile precursor fibre diameter of around 30-65 .mu.m, with a wall thickness of 5-10 .mu.m. Diameters of hollow carbon fibre in the region of 25 .mu.m from polyacrylonitrile fibres of diameters in the region of 40 .mu.m are particularly preferred. Fibre diameters are controllable through the aforementioned spinning variables. The process preferably requires stretching in a heated zone to reduce the spun fibre to the required diameter. The drawing bath conveniently contains heated liquid to facilitate this. Embrittlement that may ensue due to orientation effects and can adversely effect production of carbon fibre can be eliminated by relaxation at raised temperatures.
The conversion of the hollow PAN precursor to a hollow carbon fibre is achieved via the pyrolysing process which is used for solid carbon fibres and which will be familiar to those skilled in the art.
Another aspect of the invention provides a spinneret for manufacture of hollow polymeric fibres, and in particular hollow polyacrylonitrile precursors for carbon fibres, comprising a hollow body, a first inlet for a dope, a second inlet for a coagulant. A base plate having at least one extrusion aperture for extrusion of the dope, and coagulant injection means to inject a coagulant into extruded dope solution alignable to the centre of the or each extrusion aperture and in communication with the second inlet, such that in use a stream of dope is extruded through the or each aperture having a stream of coagulant at its centre. Each injection means conveniently takes the form of a hollow needle in communication with the second inlet and provided with an aperture at one end which can be aligned with the centre of an associated extrusion aperture.
To control the flow parameters, the injection means is preferably provided with vertical microadjustment means to control the distance between it and the extrusion aperture. Lateral microadjustment means to ensure accurate centring of the injection means over the extrusion aperture are also preferred.
At its simplest, this aspect of the invention comprises a single extrusion aperture and a single injection means. In the alternative, the base plate is provided with a number of extrusion apertures and the spinneret further comprises a number of injection means alignable to the centre of the extrusion apertures to enable multiple fibre spinning from a single spinneret. In a preferred arrangement, the spinneret has a hollow body cavity divided by an upper plate incorporating the injection means into an upper portion communicating with the first inlet and a lower portion communicating with the second inlet. The upper plate is preferably provided with a number of hollow needle-like depressions protruding towards the base plate and alignable to the centre of the extrusion apertures.